In an electric power steering apparatus the steering assist force is applied to a steering mechanism by driving an electric motor in accordance with steering torque that is applied to a steering wheel by a driver. An electronic control unit with an inverter controls the motor. The inverter feeds the motor controller with motor parameters for torque generation. The control system of synchronous motors needs position feedback in order to calculate the phase currents necessary for obtaining the desired torque with maximum efficiency.
For the detecting of rotor position mostly magneto-resistive sensors are used. A sensor chip detects the magnetic field of a permanent magnet, which is preferably round in form and mounted coaxially on a rotating shaft. This configuration is only applicable fulfilling the geometrical preconditions at a free rotor side of the electric drive. That is why this arrangement is called end of shaft (EOS). The sensors are negatively affected by a stray magnetic field caused by high motor currents, leading to undesired noises and loss of rotor position measurement accuracy. Hardware design best practices have evolved to minimize the disturbing effects but other system design constraints often prevent following them. To have a better understanding of the achieved rotor position sensor (RPS) accuracy, various measurement methods have been proposed. Hardware and software filtering are already applied to suppress higher frequency noise components and to compensate for the effect of measurement delays.
DE 10 2008 043 265 A1 discloses a method for failure compensation of electric angle measurements. The magnetic interference field, which influences the magnetic field, is decomposed into single interference vectors. After that the magnetic interference field is vectorially subtracted from a detected magnetic field vector. The determination of the vector components of the measured magnetic field involves a high calculation effort. One disadvantage of the described state of the art is that the interference compensation is based on the usage of several look-up tables, which leads to a retarded detection of compensation parameters.
Thus a need exists for an improved method for rotor position measurements in power steering systems of motor vehicles by reducing the influence of stray magnetic fields without affecting the electric motor performance. Further, a reduction of undesired noises is needed.